What are the threadlike proteins that attach to platelets in blood clotting?

fibrin

Explanation:

Fibrin is a protein that acts as a first response whenever your body is injured in order to play an important role in the blood clotting process. When any type of injury that causes blood leakage occurs and coagulation begins, an alert is then sent to the body to start producing fibrin.

In the form of long strands, each fibrin molecule intertwines to form a thin mesh around the lesion. As a result, this mesh stops the blood by capturing red blood cells, leukocytes and platelets, generating a clot and forming a kind of tampon at the injured site.

In summary, we can say that fibrin is a yarn-like protein that binds to platelets in blood clotting.